Steam turbine and generator rotors are usually constructed of low alloy steel and contain a number of projecting steeples disposed circumferentially about a disc portion of the rotor for retaining blades. During service, these steeples can experience corrosion due to steam borne corrodants and erosion or wear due to steam and debris in the turbine environment. These damage mechanisms sometimes evidence themselves in pitting and cracking of the steeples and surrounding disc area or substantial metal wastings and can lead to scrapping the entire turbine rotor at great expense to the operator.
Recent developments in welding, such as improved alloys and low heat input welding techniques, have allowed low alloy turbine rotors to be rebuilt so that new steeples may be machined into the weldment. See Clark, et al., Experiences with Weld Repair of Low Pressure Steam Turbine Rotors, 47th American Power Conference, Chicago, Ill., printed by Westinghouse Electric Corporation, Power Generation, Orlando, Fla. and Clark, et al., Development of the Techniques for High Pressure Rotor Weld Repair, Westinghouse and Mitsubishi Tech. Seminar, Takasago, Japan, July 1987, which are both hereby incorporated by reference. Despite such useful techniques designed to avert costly scrapping of the rotor, steeples reconstructed from low alloy weldments offer little or no improvement in corrosion or wear life when subject to the same environmental conditions which caused the pitting and cracking in the original rotor.
Artisans have also made rotor repairs by cutting off a defective section of a rotor and welding on a new rotor portion. See U.S. Pat. No. 4,633,554 which is hereby incorporated by reference. Such techniques also do not inherently increase the corrosion or wear performance of the repaired rotor and can result in similar costly repairs.
Hard STELLITE coatings have also been applied to turbine rotors with plasma transferred arc welding technology. See Sermatech Review, "New Services for Industrial Turbines", No. 31, (Winter, 1989), which is hereby incorporated by reference. By melting a powder in a plasma system, a strong metallurgical bond can be applied to a turbine rotor disc which has acceptable hot gas solid particle erosion resistance. While this technique is somewhat successful in reducing corrosion, very selective corrosion attacks occur in areas where the coating breaks off. The corrosion in these exposed areas will generally be worse than the corrosion on the surface of the rotor when no coating is applied at all.
Accordingly, there is a need for a longer lasting rotor which can resist pitting and cracking in the turbine environment and which can be cost effectively manufactured.